The present invention relates in general to downhole perforating gun assemblies. More particularly the present invention relates to a loading tube for holding charges for insertion into a perforating gun carrier.
A perforating gun is commonly used to form perforations in a wellbore to increase the production between the wellbore and the producing formation adjacent to the wellbore. The perforating gun may be part of a perforating gun assembly, an assembly that may include several perforating guns and other components. The perforating gun and associated assembly are selected based on wellbore and producing formation characteristics. Some of the criteria for a particular perforating gun will be the shot spacing, shot phasing and the perforating length for the perforation gun.
The perforating gun assembly is typically positioned downhole to the desired perforating depth via a wireline or tubing, as examples. The firing of the perforating gun normally involves detonating its shaped charges that create radial perforation jets when detonated to form perforation tunnels from the wellbore into the producing formation.
Each perforating gun may consist of an outer cylindrical tube, often referred to as a carrier, and a loading tube located inside of the carrier. The carrier acts like a pressure vessel for the perforating gun and the shaped charges.
One of the main functions of the loading tube is to mechanically hold the shaped charges within the carrier at a certain phasing and spacing.
In the past, the loading tube has primarily been constructed of metallic materials. In particular, the loading tubes are commonly constructed of cut round steel tubes to achieve the desired shot phasing and density. The loading tubes have commonly included either a plastic jacket to mount and hold the shaped charges to the cut metal loading tube, or have metal fingers formed by the metal loading tube to mount and hold the shaped charges.
These prior art cut metal loading tubes have several disadvantages. First, when the shaped charges are detonated, the metal loading tube expands due to the case impact and explosive gaseous expansion. As soon as the loading tube collides with the inner diameter of the carrier, the energy from the loading tube is transmitted to the carrier. The metal carrier then swells outwardly under the impact of the loading tube and may fragment into pieces. This process and interaction is disadvantageous for numerous reasons, including swelling and or deformation of the carrier resulting in sticking in the wellbore when attempting to remove the perforating gun from the wellbore, fragmentation of the carrier and/or the loading tube that may leave excessive debris that may reduce production from the well and/or cause sticking of the perforating gun in the wellbore.
Additionally, the cut metal loading tube is relatively expensive and the metallic fingers increases the cost of production. The metallic fingers further provide little shock protection for the charges during transportation or conveyance of the gun. Due to the tendency of the fingers to fail the incidence of misruns of the perforating gun are increased.
The utilization of plastic jackets for the charges provides relatively good shock protection for the charges relative to the use of “metal fingers” formed by the metallic loading tube. However, these plastic jackets add expense to the perforating gun and often leave excessive debris in the wellbore.
There have been attempts to utilize low density polystyrene as a loading tube to reduce the cost relative to cut metallic loading tubes. However, failures commonly occur due to lack of strength of the loading tube especially at temperatures above approximately 210 degrees Fahrenheit.
Therefore, it is a desire to provide a loading tube for a perforating gun that addresses disadvantages of prior perforating guns and loading tubes. It is a still further desire to provide a loading tube for a perforating gun that provides shock protection for the carried charges during transportation and conveyance. It is a still further desire to provide a loading tube for a perforating gun that minimizes the debris left in the wellbore after detonation of the carried charges. It is a still further desire to provide a loading tube for a perforating gun that facilitates ease and accuracy in mounting charges in a loading tube.